1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor to be installed in an electrophotographic image-forming apparatus, a manufacturing method of the electrophotographic photoreceptor, an image-forming apparatus including the electrophotographic photoreceptor, and a method of forming an image with the electrophotographic photoreceptor.
2. Description of Related Art
A typical conventional electrophotographic image-forming apparatus includes an electrophotographic photoreceptor (hereinafter also referred to merely as “photoreceptor”), such as an inorganic photoreceptor or an organic photoreceptor. In a typical “electrophotographic” process, the photoconductive photoreceptor is charged in a dark state by a means such as corona discharge and then is exposed to selectively dissipate only the charges on the exposed portions and to produce an electrostatic latent image, and then the latent image is developed into a visualized image with a toner composed of a resin and a colorant, such as a dye or pigment.
The organic photoreceptor is advantageous compared to the inorganic photoreceptor because it has high selectivity on a sensitive wavelength range, high film formation properties, high flexibility and transparency of the resulting film, high productivity suitable for mass production, low toxicity, and low material and manufacturing costs. Most of the current photoreceptors are accordingly organic photoreceptors.
A recent organic photoreceptor is provided with a surface layer composed of a cross-linked cured resin on one surface. This configuration can improve the wear resistance, scratch resistance, and environmental stability, leading to a prolonged service life.
The surface layer has a relatively smooth surface. In other words, the roughness of the surface layer is lower and less readily roughened compared to that of a photoreceptor including no surface layer composed of a cross-linked cured resin. Unfortunately, such a smooth surface has a larger contact area with a cleaning blade and generates increased torque, resulting in vigorous stick-slip vibrations. The photoreceptor is therefore not sufficiently cleaned. In order to solve this problem, for example, JP 2003-149995 and JP 2007-94240 disclose the application of a lubricant onto the photoreceptor surface to reduce the adhesion of the photoreceptor to the toner and the cleaning blade.
For example, a developer containing a lubricant is applied to the photoreceptor surface by developing biasing in a developing step in JP 2007-94240. Unfortunately, the photoreceptor surface sometimes does not receive a sufficient amount of lubricant. This configuration cannot sufficiently reduce the adhesion, so that the photoreceptor is not sufficiently cleaned. If the photoreceptor is charged with a roller discharging mechanism in a charging step, the photoreceptor surface readily catches corona products generated during the discharge. This configuration also cannot sufficiently reduce the adhesion, so that the photoreceptor is not sufficiently cleaned. Although the application of an increased amount of lubricant can improve the cleaning operation, the lubricant is often unevenly distributed because of nonuniform printing rates in an image, resulting in the uneven densities in the image between lubricant-rich portions and lubricant-poor portions.
In another known technique, metal oxide fine particles are added to the surface layer of the photoreceptor to improve the durability of the photoreceptor. In particular, metal oxide fine particles having low resistance and a relatively large diameter (in specific, 100 nm or larger) can achieve both the high durability and the stable potential.
In another known technique, an organic filler is added to the surface layer of the photoreceptor to improve the cleaning operation. The added organic filler can appropriately roughen the photoreceptor surface, leading to an improvement in the cleaning operation.
JP H5-224453 discloses the addition of a benzoguanamine-melamine-formaldehyde condensate to the surface layer of the photoreceptor. JP H5-181299 discloses the addition of benzoguanamine resin fine particles and/or melamine resin fine particles to the surface layer of the photoreceptor.
The organic filler such as the benzoguanamine-melamine-formaldehyde condensate has low adhesion to the toner and thus can significantly improve the cleaning operation. Unfortunately, if such an organic filler is used with metal oxide fine particles, coagulation occurs in a coating solution for preparation of a surface layer. In particular, the combination of the organic filler with metal oxide fine particles having a large diameter often causes coagulation. The coagulations in the coating solution will remain in the resulting photoreceptor, which cannot provide the expected effects. The coagulations in the surface layer lead to abnormal wear of the cleaning blade, thereby significantly impairing the cleaning operation.